Alexander Graham Bell and Sumner Tainter were granted three patents during 1880 and 1881, on a wireless communication device they called the photophone. See: U.S. Pat. No. 235,199 (Bell, 1880) for “Apparatus for Signaling and Communication, Called Photophone”; U.S. Pat. No. 235,496 (Bell & Tainter, 1880) for “Photophone Transmitter”; and U.S. Pat. No. 241,909 (Bell & Tainter, 1881) for “Photophone Receiver”. The Bell-Tainter photophone transmitter ('496) consisted of an acoustically modulated diaphragm mirror that reflected solar illumination toward an optical receiver several hundred feet away. The Bell-Tainter photophone receiver ('909) was a selenium photocell mounted at the focal plane of a parabolic reflector. The selenium photocell was connected to a set of headphones. The sender would adjust the transmitter mirror to position the solar reflection onto the receiver's photocell. The Bell-Tainter device ('496) was able to transmit crude and noisy acoustical signals along an optical communication path up to 213 m (700 ft). The photophone was largely forgotten by the scientific and engineering communities shortly after their invention, and after Bell redirected his inventive energies to his inventions relating to flight. More recently, Calvin R. Graf briefly describes a simple photophone in part of a chapter of his 1985 book entitled “Exploring light, radio & sound energy” (Blue Ridge Summit, Pa.: TAB Books; ISBN # 0-8306-0758-7). The photophone described by Graf was modified to use natural acouto-optical modulators, and would only function over relatively short distances, as with the Bell-Tainter photophone, because the Graf and the Bell-Tainter photophones did not describe or anticipate the filtering means needed for suppressing optical turbulence noise that results from long distance optical propagation in the atmosphere and they did not describe or anticipate the stereophonic and multichannel sensor systems disclosed and claimed in the present Passive Long Range Acoustic Sensor invention.
Other prior art that is only tangentially related, but worthy of mention includes laser microphones, and laser vibrometers. Parabolic microphones are also tangentially related because they are used to listen to acoustical sounds and signals over distances up to a few hundred meters or so, depending upon distinction of signal and interfering acoustical noise. Parabolic microphones operate using significantly different principles, and do not utilize an optical channel. Laser microphones and their related laser vibrometers actively transmit laser radiation to an acousto-optical modulator, and are thereby detectable and have other limitations, such as limitations in range and atmospheric conditions. The advantages of the Passive Long Range Acoustic Sensor include:    1. Passive, non cooperative, and covert operation;    2. Portability of system and ease of transporting to line-of-sight for necessary remote optical receiver link (6) between acousto-optical modulator (3) and its glints (8), and the Sensor (7) receiver system;    3. Low power operation;    4. Very long range operation that has been demonstrated in remote listening tests at 40+ kilometer (25 mile) distances;    5. Very long range acoustic and vibration measurements are possible, including target acoustical signature, airflow turbulence, target kinematics, and scene geometry;    6. Stereophonic and multi-channel operation capabilities, including beam-forming applications;    7. Use of signal processing techniques and filters to enhance detection and analysis of otherwise difficult to obtain signals.    8. Compact and portable system size for some embodiments of the system;    9. Relatively low manufacturing cost and maintenance cost of the simple embodiments of the system; and    10. Relative easy of use of the simple embodiments of the system
A significant new and useful improvement of the prior art photophone, with the advantages listed above, and adapted for many more applications and for long range acoustical sensing and analysis is the Passive Long Range Acoustic Sensor disclosed herein. The Passive Long Range Acoustic Sensor is essentially a non-coherent tristatic (triple link) optical vibrometer, capable of long range acoustic sensing. This tristatic system uses distant naturally occurring acousto-optical modulators (3), possessing glints (bright specular reflections) (8), to establish a set of long range optical receiver links (6) to the Passive Long Range Acoustic Sensor. Acousto-optical modulators (3) are functioning as optical elements capable of acoustic modulation. Examples of naturally formed, non cooperative reflective acousto-optical modulators (3) include building windows, curved vehicle windshields and shiny metal surfaces such as vehicle bodies, aircraft fuselages, storage tanks, and the like. Acoustic sources (5) near the acousto-optical modulators (3) cause weak mechanical deformations of the acousto-optical modulator's (3) membrane or surface. When properly illuminated by the sun or other intense light sources (1), acousto-optical modulators (3) cause bright glints (8) that are optically observable at large distances from a remote optical receiver link (6), with a telescope or the like. An audio signal (5) can be recovered from the observed glint by detecting the weak intensity fluctuations using a photodetector at the focal plane of a telescope, telescopic lens, or the like, and this signal may then be filtered and processed to yield desired acoustic signals and information. This new and useful technique provides very long range remote acoustic sensing with operation demonstrated by the inventor over 40 kilometer (25 mile) terrestrial links. Remote acoustic sensing from orbital distances is also likely possible, and provides a variety of aerospace and satellite diagnostic applications. Remote acoustic sensing is both passive and covert, and therefore has may be applied to sense regions that may be inaccessible or without previously installed transmission links.
The Passive Long Range Acoustic Sensor requires three separate links to function effectively. These three links are the optical illumination link (2) from the optical illumination source (1) (the sun) to the acousto-optic modulator (3), the acoustic source link (4) from the acoustic source (5) to the acousto-optical modulator (3), and the remote optical receiver link (6) from the acousto-optical modulator (3) to the remote Sensor (7). As such, a simple embodiment of the Sensor system is dependent upon the target solar aspect and its specular reflective properties. This disadvantage may, however, be overcome with use of more advanced detectors and the use of non solar, non cooperative, or artificial illumination means for one or more of the optical illumination links (2), thereby creating a hybrid embodiment of the present invention, as disclosed below and shown in FIG. 7. Another means of overcoming the aforementioned disadvantage is to optimize the geometry and material vibrational characteristics of the acousto-optical modulator (3), or to utilize an acousto-optical modulator (3) mounted onto the object to be sensed or monitored as desired. The second disadvantage related to this tristatic link requirement is the relatively low acoustic sensitivity, generally requiring relatively loud acoustic sources (5) such as horns, public address systems, crowds, vehicles and engines. Naturally occurring acousto-optical modulators (3) generally have a poor acoustic to optical modulation conversion efficiency. Furthermore, atmospheric scintillation introduces significant noise into the remote optical receiver link (6) between the acousto-optical modulator (3) and the remote optical receiver, i.e., the Passive Long Range Acoustic Sensor (7). Relatively loud acoustic sources (5) such as sirens, motors, trains, aircraft, gunshots, public address systems, playgrounds and crowds, have been detected and analyzed by the inventor, from distances as far as 40 or more kilometers, but simple embodiments of the Passive Long Range Acoustic Sensor requires loud acoustic sources (5). Acoustic sensitivity may, however, be increased by using more advanced hardware, improved signal processing techniques, and multi-channel Sensor acoustic beam forming techniques, applied to modified embodiments of the present invention.
Although there is the disadvantage of the Passive Long Range Acoustic Sensor requiring these three simultaneous and separate high quality links (2, 4, 6) to function effectively, the potential countermeasures described above, and the advantages and valuable applications of the Passive Long Range Acoustic Sensor outweigh the illumination and communication linkage problems.
Heretofore, the means used for remote acoustical sensing include parabolic microphones and laser microphones. These two means have certain advantages and limitations. Parabolic microphones have a relatively short range, and although laser microphones have a longer range than parabolic microphones, their range is short in comparison to the present Passive Long Range Acoustic Sensor. Some of the most significant limitations and disadvantages associated with these means includes but is not limited to their limited range, noise interference, power requirements for laser microphones, and laser microphones' use of the artificial and non-covert illumination means of a laser to activate an acousto-optical modulator (3). Another set of practical limitations and disadvantages are the fact that the prior art laser microphone acousto-optical sensing means requires more equipment, more costly equipment, more set-up and assembly time, more training to operate, safety issues related to the use of lasers, and the larger size and less mobility of the equipment. The simple embodiments and some modified embodiments of the Passive Long Range Acoustic Sensor are portable and mobile, and are able to be aligned to acquire a direct line-of-sight to an acousto-optical modulator (3) and its associated glint (8) or to a set of acousto-optical modulators (3) and their set of glints (8), for use with multi-channel embodiments of the Passive Long Range Acoustic Sensor.
In trying to solve these disadvantages and problems with diverse contemporary remote acoustic sensing technology, the inventor, Dan Slater, devised, invented and design engineered, the present new and useful Passive Long Range Acoustic Sensor and a set of derivative embodiments.
There are a number of new and useful applications for the Passive Long Range Acoustic Sensor described herein, and that would not be viable applications using the prior art technologies. These applications include, but are not limited to: very long range acoustic sensing, listening, and analysis, beam forming with multi-channel embodiments, and very long range vibration and turbulent airflow sensing and analysis. Analysis may include, for example, identification of the signal, diagnostics, and data gathering. More specifically, such long range listening applications may include detection, identification, and analysis of vehicles, vibrations and flutter, explosions, gunshots, public address systems, crowds and other loud acoustic sources (5). Acoustic source localization is possible when multiple remote optical receiver links (6) with audio channels are simultaneously received. This technique may be used to localize the very remote location of a gunshot or other loud acoustical source (5). Acoustic beam forming using multiple remote optical receiver links (6) with audio channels can be used to improve the sensitivity and suppress and filter unwanted acoustic interference. Another significant advantage of the present Sensor (7) system is that it allows the user to extract audio from video or high frame rate electronic or film camera imagery for simultaneous visual and aural sensing.
More specifically, the long range turbulent airflow sensing applications include detection, identification, and analysis of atmospheric turbulence, vehicle traffic and turbulence, building maintenance devices such as vent fans, and other air flow sources. Additional applications of the Passive Long Range Acoustic Sensor may be found in the fields of meteorology, aerodynamics, traffic engineering, public safety and rescue operations, remote sensing and exploration, and entertainment in the visual arts and music. The detection and analysis of building vent fans and ventilation flows may be found through the new and useful analysis of acoustic signatures of their leakage and turbulent airflow. The meteorological applications include new and useful means of measuring and analyzing winds and turbulence. The aerodynamic applications include new and useful means of measuring and analyzing aircraft and rocket vibration and flutter, flows and wake turbulence. The traffic engineering applications include new and useful means of detecting and analyzing vehicles and multiple vehicle detection, signatures, and characterization from their wake turbulence signatures, and their velocity, acceleration, deceleration, and deviation through, for example, their Doppler shift. The public safety and rescue applications include new and useful means of detecting and analyzing audio signals over a very long distance and the passive detection of distressed persons or vehicles, including terrestrial vehicles, ships or aircraft, and covert persons and vehicles, through strobes or the like, fitted with acousto-optical modulators (3), or through the use of a small, stationary acousto-optical modulator (3) aimed toward a known Sensor (7). The remote sensing and exploration applications include new and useful means of remote sensing of areas that may be inaccessible or inconvenient to visit, and remote exploration of planetary bodies with acousto-optic modulators (3) such as shiny balloons or the like, and with other specular objects. The visual arts and music applications include new and useful means of introducing new, useful and unusual varieties of sounds or active interaction into a performance or recording, or a new set of themes into science fiction screen play.
It should be noted that a direct line of sight from the Passive Long Range Acoustic Sensor to a vehicle or acoustical source is not required. The line of sight is to the glint (8) or set of glints (8), which is the source of the remote optical receiver links (6), which carry the acoustical sources (5), whether they be loud sounds or wake turbulence in the local vicinity. Doppler shifts have been detected and analyzed at distances up to 40 kilometers, using the Passive Long Range Acoustic Sensor.
Observing platforms for the Passive Long Range Acoustic Sensors described herein include stationary and mobile ground-based platforms, as well as stationary and mobile aerial and space-based platforms. The mobile aerial platforms with one or more Passive Long Range Acoustic Sensors may be mounted on observational balloons, unmanned aerial vehicles (UAVs, drones) (16) and helicopters, and airplanes, as shown in FIG. 8. These platform-based Sensors need not be stationary, but may have electromechanical means to be directed at and to follow a glint (8) or set of glints (8). Other embodiments of the Passive Long Range Acoustic Sensor may follow a moving glint (8) or a moving set of glints (8) through the use of a tripod (20) or the like, with an aiming means (21). Throughout this specification and its appended claims, the term tripod (20) is to be broadly construed, and is intended to include any means of supporting the Sensor (7) and the aiming means (21) is also intended to be broadly construed and is intended to also include aiming handgrips and automatic, optically-based electromechanical means of aiming at a glint (8) or set of glints (8), and maintaining aim, including the associated feedback and control systems.